1. Field of the Invention
The present invention relates to power conversion and to micro-electromechanical (MEM) devices.
2. State of the Art
A large proportion of present power electronics research focuses on miniaturization of power converters. The goal is to maximize efficiency and power density. Power densities of approximately 50 W/cm.sup.3 have been reported in the literature for discrete switch-mode supplies. Efforts at miniaturizing power converters often are hampered by the difficulty of integrating magnetic components with other circuit elements using existing processing technology. In the area of micromachined electrostatic sensor and actuators, for example, such sensors and actuators often require high voltages for proper operation. Currently, these voltages must be generated off-chip, because magnetic components are generally unavailable in ICs.
One interesting approach integrates an inductor with a capacitor using microstrip theory. This method permits the manufacture of integrated LC structures suitable for power conversion applications. Apparently, these devices are not compatible with existing planar integrated circuit technology. Yachi et al. ("A new planar microtransformer for use in micro-switching converters," PESC, pp. 20-26, June 1991) outlines a planar microtransformer manufactured by dry process techniques using Cu, Ta, and CoZrRe as the magnetic material. This process apparently permits integration of a microtransformer with power semiconductor devices. A problem with this device is the large series resistance of the device, apparently caused by the contact resistance between the two separate metallization steps needed to define the coil. Another approach for constructing integrated power supplies has been based on the methodology of switched capacitor circuits. Drawbacks of this approach include lack of isolation and inherent switching losses.
An electrostatic microresonant power conversion devices is described in a paper of the same name, IEEE PESC, Vol. II. pp. 997-1002, 1992, by the present inventors. As described therein, the approach followed is to replace the LC tank in a resonant converter with a micromechanical device, thereby avoiding the fabrication of magnetic components. Two micromechanical devices are coupled in tandem. Isolation and energy transfer between primary and secondary ports is achieved with an insulating mechanical coupling. The device is manufactured by planar techniques using readily available silicon processing equipment. This compatibility permits integration of the device with semiconductor devices to yield a monolithic power supply.
The foregoing approach, while effective, requires a high voltage bias supply, the conventional generation of which decreases the utility of the design. There remains a need for improved MEM-based power conversions devices.